An 802.11 station can prolong its battery life by powering off its radio when not transmitting or receiving. The efficiency of the protocol determining when a station should wake up in order to send and/or receive transmissions destined to it will depend on the type of traffic involved. The traffic may be periodic in one or both directions, and have different priorities. The 802.11e draft standard provides two mechanisms for power-saving delivery: The Power Management mechanism in the 802.11-1999 standard, referred to here as the ‘legacy’ mechanism, and the APSD (Automatic Power Saving Delivery) mechanism. The latter is designed to improve QoS performance by taking advantage of the periodic nature of VoIP and other synchronous downlink streams. The legacy mechanism can also be used for VoIP calls, but less effectively. In this paper we describe how the two mechanisms would be used for bi-directional periodic streams. We also describe how traffic prioritization would be handled.
While operating under legacy power management, a station can have its downlink traffic buffered when it is asleep. The station requests buffering of its traffic by setting the power management bit “ON” in the frame control field of a frame transmitted uplink. When it wakes up, the station can receive its frames by indicating that it is awake. This can be done in two ways: one by setting the power management bit “OFF” in the flame control field of its frames uplink, and the other by sending a PS (power saving) poll to request the transmission of a single frame. When the former method is used, the AP can notify the station when the all buffered frames have been transmitted and the buffer is empty by setting the more data bit in the last frame transmitted to a station “OFF”. If the station has no more uplink frames to send, it can go back to sleep after acknowledgement has been sent for the last buffered frame. If more frames are pending uplink transmission, the station can stay awake to transmit these frames, and will go to sleep, provided the acknowledgements received from the AP continue to indicate that there are no frames buffered. When the station goes back to sleep, it can turn the power management bit “ON” in the frame control field of either the ACK to the last downlink frame received, or in the frame control field of the last uplink frame sent. All retransmissions of frames lost or received with errors will occur during the time interval when the powersaving bit is “OFF”.
The legacy power saving mechanism can be combined with new features of the 802.11 e draft standard to yield a more efficient power saving mechanism. Specifically, the new QoS frame format allows the AP to indicate whether a QoS frame is the last frame to be transmitted in the present wake-up period by setting the last bit in the QoS control field. This flag is different from the more data bit in the frame control field, used to signal that there are more frames buffered at the AP for the destination station. The capability to indicate both is useful when the AP must postpone certain downlink transmissions in order to meet QoS requirements. A station that goes to sleep having received a downlink frame with both the Last bit “ON” and the More Data bit “ON” may send a null frame with Power Management bit “OFF” to receive the remaining buffered frames at a later time.
A power-saving station can find out whether there is buffered traffic at the AP by reading the beacon TIM, which indicates so. If, however, a station has a reason to wake up and transmit uplink frequently (that is, at time interval shorter than the beacon period), there is no need to wake tip in order to listen to the beacon TIM. The more data bit in the acknowledgement lets the station know of its buffered frames pending transmission.
An example of this arises during a VoIP call. In general, a station capable of VoIP calling is typically in one of two modes: Stand-by or Active. In the stand-by mode it is powered on but does not engage in calling. A station in stand-by mode may receive or send signaling traffic during the initiation of a call. In the active mode the station is engaged in a call, or equivalently, generates and receives traffic periodically. A station exchanging traffic with the AP that is not symmetric can operate as in stand-by VoIP mode. A station sending periodic traffic streams uplink without receiving a periodic downlink stream can operate as in the VoIP active mode, provided that the delay tolerance for downlink frames is not less than the interarrival time of the periodic uplink stream. The ‘legacy’ power saving mechanism in 802.11-1999 can be used for both modes.
If silences are not suppressed during a VoIP call, there will be frames generated periodically along both directions, uplink and downlink. In that case, the station would wake up to transmit its uplink frames. Having been notified of the existence of buffered downlink frames by the more data bit in the acknowledgement frame, the station will stay awake to receive its buffered frames.
With silence suppression at the station, there are two possible modes of operation. The station may wake up periodically and transmit null frames. Acknowledgement of uplink frames would indicate whether downlink frames are pending transmission, and hence the station would stay awake to receive its buffered frames, as described above. Alternatively, the station may listen to the beacon TIM to ascertain the same. The advantage of the second method is that less power would be used if the downlink side were silent, too. If the downlink side is sending frames, it is preferable to just send null frame at the service period, as less power would be used. The station will have omitted powering on for the TIM. The best arrangement would be a combination of the two approaches.